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Meiosis is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately result in four cells with only one copy of each chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilisation, the haploid cells produced by meiosis from a male and female will fuse to create a cell with two copies of each chromosome again, the zygote.
In cell biology, mitosis is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maintained. Therefore, mitosis is also known as equational division. In general, mitosis is preceded by S phase of interphase and is often followed by telophase and cytokinesis; which divides the cytoplasm, organelles and cell membrane of one cell into two new cells containing roughly equal shares of these cellular components. The different stages of mitosis altogether define the mitotic (M) phase of an animal cell cycle—the division of the mother cell into two daughter cells genetically identical to each other.
Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there are two distinct types of cell division: a vegetative division (mitosis), producing daughter cells genetically identical to the parent cell, and a cell division that produces haploid gametes for sexual reproduction (meiosis), reducing the number of chromosomes from two of each type in the diploid parent cell to one of each type in the daughter cells. In cell biology, mitosis (/maɪˈtoʊsɪs/) is a part of the cell cycle, in which, replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the total number of chromosomes is maintained. In general, mitosis is preceded by the S stage of interphase and is often followed by telophase and cytokinesis; which divides the cytoplasm, organelles, and cell membrane of one cell into two new cells containing roughly equal shares of these cellular components. The different stages of mitosis all together define the mitotic (M) phase of animal cell cycle—the division of the mother cell into two genetically identical daughter cells. Meiosis results in four haploid daughter cells by undergoing one round of DNA replication followed by two divisions. Homologous chromosomes are separated in the first division, and sister chromatids are separated in the second division. Both of these cell division cycles are used in the process of sexual reproduction at some point in their life cycle. Both are believed to be present in the last eukaryotic common ancestor.
Prophase is the first stage of cell division in both mitosis and meiosis. Beginning after interphase, DNA has already been replicated when the cell enters prophase. The main occurrences in prophase are the condensation of the chromatin reticulum and the disappearance of the nucleolus.
Anaphase is the stage of mitosis after the process of metaphase, when replicated chromosomes are split and the newly-copied chromosomes are moved to opposite poles of the cell. Chromosomes also reach their overall maximum condensation in late anaphase, to help chromosome segregation and the re-formation of the nucleus.
In cell biology, the spindle apparatus refers to the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter cells. It is referred to as the mitotic spindle during mitosis, a process that produces genetically identical daughter cells, or the meiotic spindle during meiosis, a process that produces gametes with half the number of chromosomes of the parent cell.
Telophase is the final stage in both meiosis and mitosis in a eukaryotic cell. During telophase, the effects of prophase and prometaphase are reversed. As chromosomes reach the cell poles, a nuclear envelope is re-assembled around each set of chromatids, the nucleoli reappear, and chromosomes begin to decondense back into the expanded chromatin that is present during interphase. The mitotic spindle is disassembled and remaining spindle microtubules are depolymerized. Telophase accounts for approximately 2% of the cell cycle's duration.
The spindle checkpoint, also known as the metaphase-to-anaphase transition, the spindle assembly checkpoint (SAC), the metaphase checkpoint, or the mitotic checkpoint, is a cell cycle checkpoint during mitosis or meiosis that prevents the separation of the duplicated chromosomes (anaphase) until each chromosome is properly attached to the spindle. To achieve proper segregation, the two kinetochores on the sister chromatids must be attached to opposite spindle poles. Only this pattern of attachment will ensure that each daughter cell receives one copy of the chromosome. The defining biochemical feature of this checkpoint is the stimulation of the anaphase-promoting complex by M-phase cyclin-CDK complexes, which in turn causes the proteolytic destruction of cyclins and proteins that hold the sister chromatids together.
A kinetochore is a disc-shaped protein structure associated with duplicated chromatids in eukaryotic cells where the spindle fibers attach during cell division to pull sister chromatids apart. The kinetochore assembles on the centromere and links the chromosome to microtubule polymers from the mitotic spindle during mitosis and meiosis. The term kinetochore was first used in a footnote in a 1934 Cytology book by Lester W. Sharp and commonly accepted in 1936. Sharp's footnote reads: "The convenient term kinetochore has been suggested to the author by J. A. Moore", likely referring to John Alexander Moore who had joined Columbia University as a freshman in 1932.
Cohesin is a protein complex that mediates sister chromatid cohesion, homologous recombination, and DNA looping. Cohesin is formed of SMC3, SMC1, SCC1 and SCC3. Cohesin holds sister chromatids together after DNA replication until anaphase when removal of cohesin leads to separation of sister chromatids. The complex forms a ring-like structure and it is believed that sister chromatids are held together by entrapment inside the cohesin ring. Cohesin is a member of the SMC family of protein complexes which includes Condensin, MukBEF and SMC-ScpAB.
Mad2 is an essential spindle checkpoint protein. The spindle checkpoint system is a regulatory system that restrains progression through the metaphase-to-anaphase transition. The Mad2 gene was first identified in the yeast S. cerevisiae in a screen for genes which when mutated would confer sensitivity to microtubule poisons. The human orthologues of Mad2 were first cloned in a search for human cDNAs that would rescue the microtubule poison-sensitivity of a yeast strain in which a kinetochore binding protein was missing. The protein was shown to be present at unattached kinetochores and antibody inhibition studies demonstrated it was essential to execute a block in the metaphase-to-anaphase transition in response to the microtubule poison nocodazole. Subsequent cloning of the Xenopus laevis orthologue, facilitated by the sharing of the human sequence, allowed for the characterization of the mitotic checkpoint in egg extracts.
Aurora kinase B is a protein that functions in the attachment of the mitotic spindle to the centromere.
Polo-like kinases (Plks) are regulatory serine/threonin kinases of the cell cycle involved in mitotic entry, mitotic exit, spindle formation, cytokinesis, and meiosis. Only one Plk is found in the genomes of the fly Drosophila melanogaster (Polo), budding yeast (Cdc5) and fission yeast (Plo1). Vertebrates and other animals, however, have many Plk family members including Plk1, Plk2/Snk, Plk3/Prk/FnK, Plk4/Sak and Plk5. Of the vertebrate Plk family members, the mammalian Plk1 has been most extensively studied. During mitosis and cytokinesis, Plks associate with several structures including the centrosome, kinetochores, and the central spindle.
Serine/threonine-protein kinase PLK1, also known as polo-like kinase 1 (PLK-1) or serine/threonine-protein kinase 13 (STPK13), is an enzyme that in humans is encoded by the PLK1 gene.
Mitotic checkpoint protein BUB3 is a protein that in humans is encoded by the BUB3 gene.
Kinetochore protein Nuf2 is a protein that in humans is encoded by the NUF2 gene.
TRIP13 is a mammalian gene that encodes the thyroid receptor-interacting protein 13. In budding yeast, the analog for TRIP13 is PCH2. TRIP13 is a member of the AAA+ ATPase family, a family known for mechanical forces derived from ATP hydrolase reactions. The TRIP13 gene has been shown to interact with a variety of proteins and implicated in a few diseases, notably interacting with the ligand binding domain of thyroid hormone receptors, and may play a role in early-stage non-small cell lung cancer. However, recent evidence implicates TRIP13 in various cell cycle phases, including meiosis G2/Prophase and during the Spindle Assembly checkpoint (SAC). Evidence shows regulation to occur through the HORMA domains, including Hop1, Rev7, and Mad2. Of note, Mad2's involvement in the SAC is shown to be affected by TRIP13 Due to TRIP13's role in cell cycle arrest and progression, it may present opportunity as a therapeutic candidate for cancers.
Syntelic attachment occurs when both sister chromosomes are attached to a single spindle pole.
Mad1 is a non-essential protein which in yeast has a function in the spindle assembly checkpoint (SAC). This checkpoint monitors chromosome attachment to spindle microtubules and prevents cells from starting anaphase until the spindle is built up. The name Mad refers to the observation that mutant cells are mitotic arrest deficient (MAD) during microtubule depolymerization. Mad1 recruits the anaphase inhibitor Mad2 to unattached kinetochores and is essential for Mad2-Cdc20 complex formation in vivo but not in vitro. In vivo, Mad1 acts as a competitive inhibitor of the Mad2-Cdc20 complex. Mad1 is phosphorylated by Mps1 which then leads together with other activities to the formation of the mitotic checkpoint complex (MCC). Thereby it inhibits the activity of the anaphase-promoting complex/cyclosome (APC/C). Homologues of Mad1 are conserved in eukaryotes from yeast to mammals.
Biorientation is the phenomenon whereby microtubules emanating from different microtubule organizing centres (MTOCs) attach to kinetochores of sister chromatids. This results in the sister chromatids moving to opposite poles of the cell during cell division, and thus results in both daughter cells having the same genetic information.
References
- 1 2 Corbett KD, Yip CK, Ee LS, Walz T, Amon A, Harrison SC (August 2010). "The monopolin complex crosslinks kinetochore components to regulate chromosome-microtubule attachments". Cell. 142 (4): 556–67. doi:10.1016/j.cell.2010.07.017. PMC 2955198 . PMID 20723757.
- 1 2 Plowman, Rebecca; Singh, Namit; Tromer, Eelco C.; Payan, Angel; Duro, Eris; Spanos, Christos; Rappsilber, Juri; Snel, Berend; Kops, Geert J. P.L.; Corbett, Kevin D.; Marston, Adele L. (2019-09-01). "The molecular basis of monopolin recruitment to the kinetochore". Chromosoma. 128 (3): 331–354. doi: 10.1007/s00412-019-00700-0 . ISSN 1432-0886. PMID 31037469.
- ↑ Corbett, Kevin D.; Harrison, Stephen C. (2012-06-28). "Molecular Architecture of the Yeast Monopolin Complex". Cell Reports. 1 (6): 583–589. doi: 10.1016/j.celrep.2012.05.012 . ISSN 2211-1247. PMC 3494995 . PMID 22813733.
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